The number of stereoisomers possible for 1,2-dimethyl cyclopropane is:

We need to find the number of stereoisomers possible for 1,2-dimethylcyclopropane.

1,2-dimethylcyclopropane has two methyl groups attached to adjacent carbon atoms (C-1 and C-2) of a cyclopropane ring. The two carbons bearing the methyl groups are each bonded to four different groups, making them stereocentres.

For a cyclic compound with two stereocentres bearing similar substituents, we consider two cases based on the relative orientation of the methyl groups:

cis-1,2-dimethylcyclopropane: Both methyl groups are on the same side of the ring. This molecule has a plane of symmetry passing through C-3 and the midpoint of the C1-C2 bond. Due to this internal symmetry, the molecule is a meso compound — it is optically inactive despite having two stereocentres. This gives 1 stereoisomer.

trans-1,2-dimethylcyclopropane: The methyl groups are on opposite sides of the ring. This molecule has no plane of symmetry. The two possible arrangements — $$(R,R)$$ and $$(S,S)$$ — are non-superimposable mirror images of each other, forming a pair of enantiomers. This gives 2 stereoisomers.

Total number of stereoisomers = 1 (meso/cis) + 2 (trans enantiomers) = 3.

The correct answer is Option D: Three.